The present invention relates to bridge-type circuits using electrically-heated sensing elements for gas detection, and more particularly to means for altering the output signal in a predetermined manner during the warmup period following initial energization of the instrument.
Instruments of this type use an electrically-heated temperature-sensitive resistance element or filament as a sensing element, and observe changes in electrical resistance of the sensing element to detect gases or to measure such quantities as concentration of combustible gases. The sensing element is connected in a bridge circuit so that the change in resistance is detected as a voltage which can be applied to a suitable meter to read the desired quantity or, if desired, can be used to actuate an alarm or other signal. When such a heated sensing element is initially energized, a certain time is required for the filament to become heated and for its temperature to stabilize at the normal operating level. Any readings taken during this time, or any signals, may be erroneous or inconsistent because of the fact that the sensing element has not reached a stable equilibrium temperature. For this reason, it is desirable to prevent use of the instrument during this initial warmup period.
It has been proposed, as shown in German Pat. No. 1,263,329, to inhibit use of the instrument during the warmup period by switching a capacitor across the meter charged with opposite polarity to that of the bridge. This arrangement prevents the meter from giving a positive indication since it tends to cause the meter to deflect in the reverse direction to its normal indication. The capacitor is normally charged by the battery voltage which also energizes the bridge, and when the instrument is switched ON, the capacitor is switched across the meter by suitable control means such as a transistor. This prevents any initial positive swing by the meter, and inhibits use of the instrument since the meter does not give a normal reading. After the capacitor has discharged, the meter returns to normal use while the capacitor is reconnected to the battery. The discharge time is determined by the capacitance of the capacitor and the internal resistance of the meter, together with any necessary series resistance, and since a relatively long time constant may be needed, a high capacitance is required because the internal resistance of the meter is usually quite low. The required large capacitance makes it necessary to use an electrolytic capacitor which is undesirable. Another disadvantage of this arrangement is that accurate adjustment of the time period is not normally possible.